CHEMICAL WORK. 139 



if it takes place in a bed of calcite after its consolidation, would cause frac- 

 tures, or make the rock porous and thus capable of holding much mineral 

 oil (page 134), as in the Findlay oil region of Ohio. 



Concretionary Consolidation. 



The methods of consolidation that have been mentioned in the preced- 

 ing pages are (1) by calcareous waters ; (2) by ferruginous waters ; (3) by 

 siliceous solutions. Limestones, and rocks only partly calcareous, have been 

 consolidated almost solely by the first of these methods. The second method 

 is feeble in its results, and occurs in gravel deposits. Rocks that are colored 

 by iron oxide, and oppear to have a ferruginous cement, have usually been 

 solidified by the third method. 



Consolidation is often commenced or attended with concretionary consoli- 

 dation, or accretion around centers throughout the mass, as illustrated on 

 page 97. Isolated concretions often form in deposits of earth, clay, or other 

 material, when they contain disseminated calcareous grains (derived from 

 gi'ound shells, or any other source). Percolating waters, aided by the car- 

 bonic or humus acids which such waters are likely to contain, dissolve the 

 grains and deposit the material, in a drying time, around grains, or any 

 small object, as a nucleus. In like manner, concretions of limonite and 

 iron carbonate are made, if any ferruginous grains or any decomposable iron- 

 bearing mineral is present. Occasionally other materials make disseminated 

 concretions. 



The form of the concretion is not owing to any central control of the 

 molecular deposition, but to the regular progress of the superficial accretion, 

 and to the rate of supply of the mineral solution in vertical and horizontal 

 directions, together with the shapes of the nuclei. 



The growth of the concentric forms above described is peripheral. There 

 is also centripetal consolidation, or from the exterior inward. It commences 

 outside, owing to outside evaporation and the consequent deposition of the 

 concreting agent. The agent is commonly ferruginous. This process of 

 outside drying is exemplified by the drying away of a spot of milk two 

 inches or so in diameter on a slab of stone (as observed by the author) : the 

 evaporation goes on at the outer margin, and makes there the first ring, 

 capillary attraction inside of this ring contributing material toward it ; 

 this outer ring completed,- another ring begins and forms at the new outer 

 margin of the milk-spot ; and so ring after ring forms, until the spot of 

 milk is reduced to a series of whitish rings. On the same principle, shell 

 after shell may form in a sand-bed penetrated with a ferruginous solution, 

 because drying is gradual from the outside ; or there may be a single outer 

 shell, with loose sand inside ; or a central ball in the loose sand. The center 

 of the concretion may originally have been a piece of the decomposing iron- 

 bearing mineral which afforded the ferruginous solution. 



The concentric rings of ferruginous coloration in Fig. 141 had probably 



